Electronic devices, such as flat panel displays and integrated circuits, commonly are fabricated by a series of process steps in which layers are deposited on a substrate and the deposited material is etched into desired patterns. The process steps commonly include plasma-enhanced chemical vapor deposition (CVD) processes, thermal (non-plasma) CVD processes, and plasma-enhanced etch processes.
The substrate generally is mounted on a susceptor (alternatively called the chuck or workpiece support) within a vacuum chamber referred to as a process chamber. CVD and etch processes commonly require the substrate to be at an elevated temperature, in which case the susceptor is heated by some means such as resistive or radiant heating. In plasma processes, the plasma supplies additional heat to the substrate and the susceptor.
A process gas mixture commonly is dispensed into the process chamber through a gas distribution plate, commonly called a showerhead or showerhead, that is perforated with hundreds or thousands of orifices or passageways. The showerhead generally has a flat or slightly curved lower surface positioned close to and parallel with the upper surface of the substrate (and susceptor), and the gas passageways are distributed across the surface of the showerhead, so that the process gas dispensed through the showerhead is uniformly distributed over the area of the substrate (and susceptor).
In plasma processes, electrical or electromagnetic power is coupled to the process gas to excite it to a plasma state. The plasma decomposes the gas mixture into ion species that perform the desired deposition or etch process. In capacitively excited plasma chambers, the plasma is excited by RF power applied between the showerhead, which functions as an anode electrode, and the susceptor, which functions as a cathode electrode. An example of a showerhead in a plasma chamber is described in commonly-assigned U.S. Pat. No. 4,854,263 issued Aug. 8, 1989 to Chang et al.
It is desirable to maintain the showerhead at a high temperature comparable to that of the susceptor so that the showerhead does not cool off the susceptor. Older showerhead designs mounted the showerhead to the wall of the process chamber using relatively thick mounting flanges that kept the showerhead undesirably cool by conducting heat from the showerhead to the relatively cool wall of the process chamber. In contrast, commonly-assigned U.S. Pat. No. 6,477,980, issued Nov. 12, 2002 to White et al., and commonly-assigned U.S. Pat. No. 6,772,827, issued Aug. 10, 2004 to Keller et al., describe an improved showerhead suspension having thin suspension walls with high thermal impedance so that heat absorbed by the showerhead from the heated susceptor and the plasma is retained in the showerhead, thereby achieving a showerhead temperature comparable to that of the susceptor.
The aforesaid U.S. Pat. Nos. 6,477,980 and 6,772,827 further describe the suspension walls as being flexible to accommodate thermal expansion of the heated showerhead. For example, both patents describe a rectangular aluminum showerhead suspended by four suspension segments (suspension walls) respectively connected to the four sides of the showerhead, where each suspension wall is a rectangular aluminum sheet. Each sheet is thin enough to be flexible, so that it readily bends to accommodate thermal expansion of the showerhead in a direction approximately perpendicular to the surface of the sheet.
The aforesaid U.S. Pat. No. 6,772,827 (FIGS. 5-7 and 17) describes an additional improvement in which the each suspension wall is not rigidly attached to the showerhead, but is attached by means of pins projecting downward from the rim of the showerhead that engage with corresponding slots in a bottom flange of each suspension wall. The slots are larger than the pins so as to permit each suspension wall to slide relative to the showerhead in a horizontal direction parallel to the plane of the suspension wall, i.e., perpendicular to the direction in which the suspension wall flexes. As described in the patent, permitting such sliding is useful to accommodate rapid thermal contraction of the suspension when the chamber lid is opened to atmosphere, which causes the suspension to cool much more rapidly than the more massive showerhead.
However, Applicant has discovered that when the temperature of the showerhead and suspension bottom flange exceed about 220 degrees C., the stiction of the aluminum sometimes prevents the suspension walls from sliding relative to the showerhead. Therefore, if the chamber lid is opened when the suspension is hot, the suspension may experience thermal shock as the suspension rapidly cools and contracts while the bottom flange remains stuck to the showerhead.
Furthermore, even when the pins and slots successfully avert stress between the suspension and the showerhead, the pins and slots do not avert potentially damaging stress within the suspension wall due to rapid changes in the temperature differential between upper and lower portions of the suspension wall. Applicant has discovered that such rapid change in temperature differential commonly occurs when a hot suspension is suddenly cooled. Such rapid cooling can occur when a high temperature process step, such as a thermal CVD or plasma process step, is immediately followed by a low temperature step, such as a chamber purge step. Accordingly, there is a need for an improved design that reduces thermally induced stress in the suspension wall.